The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Electric Arc Furnace dust (“EAF dust”) is produced as fume from operating an electric arc furnace during steel making. This dust has to be collected and treated or disposed of in some way. Disposal of EAF dust by stabilization and burial is widely practiced. In many markets this is costly. Economic treatments to recover contained zinc in EAF dust has been a challenge to industry for many years.
A number of leach-recovery processes have been previously proposed to recover the zinc content from EAF type dust.
U.S. Pat. No. 3,849,121 (Burrows) teaches a method for the selective recovery of zinc oxide from industrial waste in which the waste material is leached with an ammonium chloride solution at elevated temperatures to provide a pregnant leach solution. Other metals such as iron are separated out using oxidative treatments and a zinc metal cementation step. The solution is then cooled to precipitate a mixed zinc compound which includes small amounts of zinc oxide, hydrated zinc phases which can include hydrates of zinc oxide and zinc hydroxide, as well as other phases and a large amount of zinc dichlorodiammine Zn(NH3)2Cl2 and other similar zinc-chlorine compounds. This product requires further processing to provide a commercially viable product.
U.S. Pat. No. 5,464,596 (Myerson 1) teaches an alternative method for the recovery of zinc oxide. In this method, EAF dust is leached with a lixiviant comprising 23% ammonium chloride at 90° C. The resultant pregnant liquor is treated to displacing undesired metal ions from the solution using zinc metal, and then cooled to precipitate out zinc compounds as a zinc dichlorodiammine (ZDC) crystal precipitate. The precipitate is washed to remove various soluble zinc compounds, leaving zinc oxide of greater than 99%. U.S. Pat. No. 5,759,503 (Myerson 2) teaches an improved process over Myerson 1 in which the recovery of zinc oxide from the pregnant liquor is improved by dissolving zinc oxide in an intermediate, diluting the intermediate by a factor of 3 to 30 by adding 70 to 100° C. water, and filtering out the resultant zinc oxide crystals.
None of these prior zinc recovery processes have been successfully commercialized. It is considered that the operating costs for each of these processes are high due to the need for large quantities of energy and/or the use of large quantities of sodium hydroxide in the recovery process. There may also be problems with product purity due to the presence of residual chloride from the leach process.
The main energy consumer in these prior processes is water evaporation steps which regenerate the leach solution for recycle. Substantially all of the water added for dilution and/or in hydrolysis has to be removed from the process circuits of these prior processes. The amount of water involved can be as high as 40 tonnes of water per tonne of zinc oxide. Evaporative water processes are necessary because of the high ionic strength of the concentrated solutions. These concentrations provide limited scope to use the lower energy reverse osmosis systems. Evaporative process steps are not economically attractive in a high energy cost environment.
Alternative processes which use less process water have been proposed to extract metallic products such as zinc from smelter dust.
U.S. Pat. No. 5,234,669 (Bartlett) describes a process for recovering non-ferrous metals from smelter flue dust in which smelter dust is mixed with hydrated lime, formed into agglomerates, and roasted at an optimal temperature of about 650° C. to form oxidized arsenic and sulfur which react with the lime in the agglomerates to form non-leachable compounds. The roasted agglomerates are contacted with a basic lixiviant comprising dissolved ammonia and an ammonium salt, such as ammonium chloride, to dissolve non-ferrous metals such as copper from the roasted agglomerates. The resulting pregnant leach solution is heated to vaporise the ammonia and precipitate basic compounds of copper, zinc and nickel therefrom. One lixiviant used is disclosed as comprising 4 M ammonia and between 1 to 4 M ammonium chloride. No further process steps are taught to refine this precipitate to a useable industrial product.
European patent publication EP061477A1 (Scegi SARL) teaches another a process for recovering non-ferrous metals from smelter flue dust in which the smelter flue dust undergoes ammoniacal leaching with an ammoniacal lixiviant at a temperature of between 20 to 100° C. The ammoniacal lixiviant is a 0.5 to 10 moles/liters solution of NH3/NH4Cl, NH3/(NH4)2SO4, or NH3/(NH4)2CO3. One particular lixiviant taught comprises 17 g/l ammonia and 53 g/l ammonium chloride having a temperature of between 20 to 40° C. After leaching, the undissolved residues are separated from the pregnant leach solution. Ammonia is then desorbed from the pregnant leach solution using an air stream to lower the pH of the solution from between 9 and 10 to between 4 and 7. Ammonia desorption leads to the direct formation of a metallic precipitate of hydroxide. The hydroxide can then be dehydrated after washing, using any conventional manner, to obtain metal oxide of a high purity (˜98%). No detailed process steps are taught to refine this precipitate to a useable industrial product.
Each of Bartlett and Scegi SARL provide general guidance in appropriate leaching conditions to solubilise the zinc content of smelter flue dust. However, neither provides suitable guidance in the extraction conditions or process steps in obtaining a usable industrial zinc product.
It would therefore be desirable to provide an improved or at least alternative process which leaches and solubilises zinc from a variety of sources of zinc containing materials associated with other metal oxide(s) and/or “mixed-metal” oxides. This method preferably produces zinc oxide using ammonium chloride based leach liquor using a process that has a lower energy usage than prior processes but can also produce a fed material suitable for further processing to produce zinc metals and/or other zinc compounds.
It should be understood that any metal such as zinc, manganese, lead etc should be understood to include any chemical form (i.e. metal, salts, complexes, chelates, etc) or ionic form.
It is also to be understood that all the concentrations used in this specification are based around g/KgH2O or g/L H2O and not g/L of actual liquor volume. It is to be appreciated that g/l of actual liquor volume does not really reflect the chemistry of the process of the present invention as the NH3 and high levels of salts all affect the volume especially the NH3 when present at high values.